The present invention relates to polypropylene (PP) and copolymers of propylene with a minor amount of a copolymerizable alpha-olefin monomer such as ethylene (P-co-E), individually and collectively referred to herein as "propylene polymer" for convenience "PP", which, upon exposure to gamma-radiation, develops very little objectionable color and maintains its physical properties without substantial degradation thereof. More particularly, the invention relates to articles before and after they are exposed to a preselected level of gamma-radiation.
Recent advances in PP technology have been instrumental in propagating use of the polymer for shaped articles for a wide variety of uses. The chemical inertness and lack of toxicity of PP, its low weight, and the relatively low cost of producing such articles of arbitrary size and shape, makes them peculiarly well-adapted for use in the medical or health maintenance field. Thus, PP is the material of choice for laboratory ware, including trays, funnels, Petri dishes and the like, and a variety of disposable articles including blow-molded IV-bottles, hypodermic syringes, needle shields, surgical gowns and the like. Such articles were routinely sterilized with ethylene oxide but its toxicity resulted in its gradual withdrawal as a sterilant. Ethylene oxide has been replaced by exposure to gamma radiation having an energy level in the range from about 0.5 to about 5 Mrad (megarads) for a short period of time, from about 1 minute to about 12 hours, the higher the intensity of radiation, the shorter the time required.
The problem is that PP degrades rapidly when so sterilized, developing an objectionable yellow color, and suffering a severe loss of physical integrity, the higher the intensity of radiation, the worse the degradation. The degradation, referred to as oxidative degradation, is particularly noticeable upon storage of a sterilized article at room temperature, the longer the storage period, the worse the degradation.
Considerable effort has been devoted, in the recent past, to the study of the details of the mechanisms and kinetics of the oxidation, with the expectation that such understanding will lead to the correct choice of an effective stabilizing additive (stabilizer) which will control the chemistry responsible for degradation. In an article titled "Recent Developments in the Oxidative Degradation of Polypropylene by Gamma Radiation" by Wiles, D. M. and Carlsson, D. J., theoretical equations are presented to explain the mechanisms and kinetics. They concluded that gamma radiation of PP results in theral oxidation which is qualitatively very similar to oxidation induced by other means, e.g. to photo-oxidation. The same reactive species are involved and although they will be distributed rather differently, the same oxidation products are formed.
The difference in the frequency of the wavelengths of uv light and gamma radiation, coupled with the great difference in their energy levels, makes their relative effects upon a polymer unpredictable. The differences go far deeper than the expected differences relating to their relative penetration into the polymer, as evidenced by the fact that some structurally similar compounds are far less effective stabilizers than others which appear to have all the necessary perquisites of the more effective stabilizers.
Notably, the effects of gamma radiation on PP are readily distinguishable over the effects of such radiation on other polymers (polystyrene "PS", say). PS has been found to be more stable (see "Stabilizers in gamma-irradiated polypropylene" by Horng, P. and Klemchuk, P., Plastics Engineering April '84, pp 35-37), as are several other polymers. They concluded that the high susceptibility to gamma-degradation decreed that the fate and effectiveness of stabilizers can only be assessed by determining whether they are being consumed in the irradiation process or being simply decomposed by gamma radiation. Further, they developed an experimental technique to evaluate the stability of four major genera of additives, namely an antioxidant (AO) such as tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, a hindered amine (HA) such as 2,2,6,6-tetramethyl-4-piperidyl sebacate, a phosphite such as tris(2,4-di-tert-butyl phenyl)-phosphite, and a thioester such as distearyl thiodipropionate. Their study indicated that the mechanical and molecular weight data confirmed that the residual AO or HA in the irradiated PP maintained the physical integrity of the samples by reacting with the propagating radicals and/or by decomposing peroxides. The phosphite and thioester additives were less effective because they behaved as secondary stabilizers and were completely consumed during radiation. But, they offered no basis for choosing an effective HA or AO, other than by repetitive trial and error.
Tests for color development in gamma-irradiated PP shows that HA alone did not cause significant discoloration and was the leading choice for medical end-products which were color or appearance-sensitive. Thus, HA was the best overall stabilizer for PP gamma-sterilizable products. Again, however, there is nothing to indicate how to choose the more effective among HAs, other than trial and error.
Yet, British Patent No. GB 2,043,079 discloses that PP was stabilized against gamma-radiation by incorporating a phenolic AO containing an isocyanurate group, preferably in conjunction with a thiosynergist and a stearate, clearly contradicting the foregoing conclusions.
We have now found further evidence that hindered amines with a specific structure, which amines exhibit excellent uv stabilization in numerous host polymers without notably distinguishing one polymer from another as far as their relative susceptibility to uv stabilization is concerned, are surprisingly effective to stabilize PP against degradation by gamma-radiation. Moreover, that these amines are more effective when used without an AO, phosphite or thioester.
The peculiarly distinguishing structural feature of such hindered amines is that they contain, as an essential portion of their basic structure, a polysubstituted (hereafter also referred to as "substituted" for brevity) piperazin-2-one (PSP) having an N.sup.1 -adjacent carbonyl, and at least the C.sup.3 (carbon atom in the 3-position in the ring) has two substituents (hence "polysubstituted"), which may be cyclizable, that is, form a cyclic substituent.
Though such compounds, referred to in U.S. patents listed herebelow, were known to be excellent UV stabilizers in colorless organic materials when used in combination with antioxidants, there was nothing to suggest that their incorporation in PP, alone among other polymers tested, in the absence of an antioxidant, and preferably also in the absence of a phosphite or thioester stabilizer, would provide effective stabilization against gamma-radiation.